Introduction
The determination of aqueous ion concentrations has application in many fields such as water treatment and medicine. For example, the measurement of sodium and potassium levels in blood are important aids in diagnosis of numerous conditions.
The use of compounds and compositions which selectively isolate ions from a sample solution provide a variety of methods for measuring ion concentrations. These compounds, known as ionophores are capable of selectively forming a complex with a particular ion in an hydrophobic environment to the substantial exclusion of other ions. The use of ionophores has lead to five basic analytical approaches: ion selective electrodes, liquid/liquid partitioning, fluorescence enhancement, chromophore-labeled ionophore conjugates and test strips.
Ionophores include multidentate cyclic compounds which contain donor atoms in their cyclic chains. Such multidentate compounds can be mono- or polycyclic.
Monocyclic multidentate compounds which contain donor atoms which are electron rich and which are capable of complexing with particular cations are known as coronands. Typical coronands are crown ethers in which the monocyclic chain contains oxygen as the donor atom.
The polycyclic analogs of coronands are known as cryptands. Cryptands include bi- and tricyclic multidentate compounds. The cyclic arrangement of the donor atoms in cryptands is three dimensional and as a result, cryptands are capable of virtually surrounding a cation.
A third class of ionophores are known as podands. Podands are open chain ion specific compounds which, for example, contain a regular sequence of electron rich atoms such as oxygen.